There are a number of industrial processes in which it is necessary to separate NH.sub.3 from mixtures of other gases, or from aqueous streams. Perhaps the largest scale separation is the removal of NH.sub.3 from the gas mixture that is present in the recycle loop of an ammonia synthesis plant. Currently, this separation is accomplished by refrigeration, with ammonia being removed in a liquid state. In other operations, such as in petroleum refineries and other related industries, ammonia is removed by steam stripping. Various attempts have been made to develop an efficient and practical sorption system for the selective removal and recovery of NH.sub.3 from various gaseous and aqueous streams.
Alumina silicate zeolites and high surface area carbons, have been widely used as ammonia sorbents. U.S. Pat. No. 4,537,760 and U.K. patent application 2145702-A both disclose processes which utilize such sorbents for the separation of NH.sub.3 from the mixture of gases present in the recycle loop of an ammonia plant. Additionally, R. D. Rice and J. V. Busa in Chemical Engineering Processing, October 1984, page 61 disclosed the use of solutions of ammonium diacid phosphate as selective, reversible sorbents for ammonia.
A number of organic polymer systems have been shown to sorb ammonia from gaseous mixtures. S. Kamata and M. Tashiro, J. Chem. Soc. Jpn., Ind. Chem. Soc., 73 1083 (1970) disclose the use of cation exchange resins in the proton and metal ion forms to take up ammonia with varying degrees of reversibility. It was shown that the H.sup.+, Ni.sup.II, Cu.sup.II, Co.sup.II and Zn.sup.II forms of Amberlyst 15 cation exchange resins have an appreciable capacity for NH.sub.3. Similarly, Z. Prokop and K. Setinek, J. Polym. Science, Polym. Chem. Ed. 12, (11), 2535-43 (1974) disclose the use of cation exchange resins in the ammonium (NH.sub.4.sup.+) form for absorption of ammonia. Other known materials which can be used for ammonia absorption include cellulose acetate, ethyl cellulose, cellulose acetabutyrate and hydroxycelluloses. Recently, C. H. Lochmuller, et al. Analytical Letters 18(A)(4) 423 (1985) have reported the use of Co.sup.2+ ion exchanged Nafion (available in the Na.sup.+ form from E. I. DuPont & Company) as a reversible sorbent for NH.sub.3.
It was disclosed by H. W. Foote and M. A. Hunter, J. Am. Chem. Soc., 42, 19 (1920) that ammonia can be sorbed by ammonium thiocyanate. It has been known for many years that ammonium nitrate and ammonium thiocyanate deliquesce in the presence of ammonia vapor, yielding very concentrated solutions of these salts in liquid ammonia. Recent studies, using Raman spectroscopy, have shown that there is a strong interaction between NH.sub.3 and the sulfur end of the SCN.sup.- ion, although the mechanism of this interaction is not fully understood. Foote and Hunter disclosed the possibility of using ammonium thiocyanate as an absorbent to effect the removal of NH.sub.3 from the recycle loop of an NH.sub.3 plant, but implementation was not accomplished presumably because of the highly corrosive nature of the ammonia/NH.sub.4 SCN solutions towards most metals.
Several membrane-based gas separation technologies for the separation of ammonia from other gases have received limited disclosure in the literature. Most of the polymeric materials discussed in the prior art which are used for membrane separations, exhibit poor selectivity for ammonia passage and hence are not well suited for ammonia separation processes. Other difficulties with such polymeric membranes are related to the low absolute permeability of NH.sub.3 and the stability of the polymers to NH.sub.3 at higher than ambient temperatures.
D. W. Brubaker and K. Kammermeyer, Ind. Eng. Chem., 46,733 (1954) disclosed the use of a polyethylene film membrane for the separation of NH.sub.3 from N.sub.2 and H.sub.2 in an ammonia synthesis plant. Because of difficulties associated with the low absolute permeability of the film for NH.sub.3, and the poor separation factor for NH.sub.3 versus H.sub.2 and N.sub.2, it was felt that the system would not be competitive with the conventional refrigeration methods.
U.S. Pat. No. 3,545,931 discloses the use of a polytetrafluoroethylene membrane for NH.sub.3 separation in conjunction with a system which detects and quantifies the concentration of NH.sub.3 in aqueous streams. Although actual separation data is not given in the patent, the permeability of the polytetrafluoroethylene membrane towards NH.sub.3 is very low as it is for other gases.
Kostrov, et al., Plast. Massy, Vol. 5, pp. 18-19, (1981) found that a vinylidene fluoride-tetrafluoroethylene copolymer membrane exhibited surprisingly high NH.sub.3 selectivity/permeability properties, especially in relation to the parent homopolymers. The high permeability of ammonia is ascribed to an unspecified reaction between ammonia and the copolymer, which is rendered yellow upon exposure to NH.sub.3 gas. lt is known that polyvinylidene fluoride is degraded on prolonged contact with aniline, which is a much weaker base than ammonia, at 23.degree. C. and by aqueous NaOH at 120.degree. C. It is, therefore, believed that the above copolymer and its NH.sub.3 reaction product would ultimately degrade with usage and, therefore, would not be a viable material for permeating ammonia.
Russian Pat. No. SU 1063774-A references unpublished Russian data on a hydrated cellulose membrane for the separation of NH.sub.3 from H.sub.2, and claims improved membranes based on aromatic polysulfonamide polymers. Hydrated cellulose has a moderate NH.sub.3 permeability and selectivity performance in separating NH.sub.3 from H.sub.2, but it has a narrow range of usefulness with respect to temperature and pressure of ammonia. Degradation of the membrane is observed at higher temperatures and over extended use. R. M. Barrer, et al., J. Chem. Soc. Faraday Trans. 1,69, 2166 (1973) describe the operation of a very selective carbon plug membrane for the separation of NH.sub.3 from N.sub.2 and H.sub.2. The membrane, which relies on the condensation and "surface flow" of NH.sub.3 in the microporous carbon, has been shown to operate effectively with an NH.sub.3 /H.sub.2 selectivity of about 250 at conditions that are near the point of liquefaction of ammonia. Otherwise, however, the NH.sub.3 /H.sub.2 selectivity decreases rapidly at other conditions. Accordingly, it is unlikely that this carbon membrane would be useful for the separation of NH.sub.3 from other gases in the ammonia synthesis loop at the conditions of interest.
Recently S. Kulprathipanja and S. S. Kulkarni have disclosed in U.S. Pat. No. 4,608,060 the preparation of a multicomponent membrane comprising silicone rubber and polyethylene glycol which can separate ammonia from N.sub.2 and H.sub.2. The membrane shows a very high ammonia permeance of .apprxeq.30.times.10.sup.-5 cm.sup.3 (STP)/cm.sup.2.sec.cmHg, but the NH.sub.3 /H.sub.2 separation factor is relatively low (.apprxeq.80).
G. P. Pez and R. T. Carlin have described in European Patent Application No. 86102208.5, the achievement of an effective separation of NH.sub.3 from N.sub.2 and H.sub.2 at high temperatures (250.degree.-350.degree. C.), using a membrane consisting of a reversibly ammonia reactive molten salt (e.g., ZnCl.sub.2) immobilized in a porous metallic or ceramic support.